assistance with making swale calculations

So I got some heavy clay soil on site. The wettest year on record was 1940-41 with about 32 inches of rain. there was one year that 3 inches of water fell in a day. there is about a 6.25 percent grade across 289 feet. in some places it is a little bit steeper.

I am trying to think of more information that I need to take into consideration before the build.